1. Field of the Invention
The invention relates to wear resistant coatings and, more particularly, to wear resistant coatings for piston rings used in diesel engines.
2. Description of the Prior Art
Plasma deposited coatings normally provide good wear resistance in internal combustion engines. A variety of compositions are used to extend the wear resistance of the ring face which is in sliding contact with the cylinder wall or cylinder liner. This is usually accomplished by using blends of powders containing relatively large percentages by weight of carbides or oxides. These constituents in the coating have generally increased the wear resistance of the ring face, but in many instances they have caused an increase in the wear on the cylinder wall or cylinder liner. An analysis of many piston rings and cylinder liners after engine tests indicate that the high wear rates are predominately attributed to the aggressive nature of the carbides or metallurgical deterioration of the wearing surface which introduces debris into the engine. Therefore, while ring life improved, liner wear has become unacceptable. The coating disclosed in our U.S. Pat. No. 3,556,747 consists of an admixture of primary particles of chromium carbide, nickel chromium and molybdenum. In tests designed to improve the adhesion of the coating, thereby preventing the introduction of debris, the composition of nickel chromium and molybdenum was increased while the chromium carbide content was lowered. Surprisingly, engine test results of the modified coatings had lower wear rates on both the rings face and cylinder liner.